Optical film

ABSTRACT

An optical film comprises a transparent supporting substrate and a structuralized layer integrally formed on the supporting layer and having a plurality of light-concentrating units including design that varies in height along their length or varies in pitch of prism structure to overcome the optical defects (wet-out) of the optical film and to enhance the optical properties of the optical film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional PatentApplication No. 61/692,199 filed on Aug. 22, 2012. This application isfully incorporated by reference as if fully set forth herein. Allpublications noted in the specification below are fully incorporated byreference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical film, and, in particular,relates to an optical film capable of avoiding optical defects (e.g.wet-out, moire fringes and so on). Further, the present inventionrelates to an optical film used to the backlight module of a flat paneldisplay.

2. Description of Related Art

The backlight module of the flat panel display and its common structureare very familiar to a person with ordinary skills in the art. Most ofthe current backlight modules used in the flat panel display employ twolight enhancement sheets in order to have better output brightness.However, a troublesome optical coupling effect (i.e. wet-out) morefrequently occurs when the two stacked light enhancement sheets getclose to each other.

The convention methods for reducing the occurrence of the wet-out letthe two optical films physically away from each other as far aspossible. That is, the heights of adjacent portions of the two opticalfilms are accordingly modified. U.S. Pat. No. 5,771,328 disclosed anoptical film having taller prisms (See numeral reference 56 in itsdrawings) and shorter prisms (See numeral reference 54 in its drawings).When the optical film is disposed on another one, the physical proximityto another is limited so that the visibility of a wet-out pattern isreduced. However, the taller prisms of the prior art still may be visualas straight lines through the optical film.

In view of above, since the applicant of the present application havefound the shortcomings of the foregoing prior art for a long term, heproposes a novel optical film so as to overcome the shortcomings.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an optical film with a structuralizedsurface. The structuralized surface can get rid of the occurrence ofoptical defects in a display, and particularly, random or inhomogeneousoptical characteristics can avoid the defects such as wet-out and moirefringes.

The present invention provides an optical film comprising a transparentsupporting substrate and a structuralized layer. The structuralizedlayer is integrally formed on the supporting layer, and has a pluralityof light-concentrating units. The light-concentrating units includesdesign that varies in height along their length or varies in pitch ofprism structure to overcome the optical defects (e.g. wet-out) of theoptical film and to enhance the optical properties of the optical film.

The present invention provides an optical film, comprising: atransparent supporting substrate; a structuralized layer including aplurality of prisms integrally arranged on the transparent supportingsubstrate along their longitudinal directions; wherein the height of atleast one of the prisms varies along its longitudinal direction and thevariation in the height includes a first change mode and a second changemode; wherein the trajectory generated from the second change mode issuperposed on and along the trajectory generated from the first changemode.

The present invention provides an optical film, comprising: atransparent supporting substrate; a structuralized layer including aplurality of prisms integrally arranged on the transparent supportingsubstrate along their longitudinal directions; wherein the pitchesbetween the prisms vary with a first change mode and the heights of theprisms vary with a second change mode along their longitudinaldirections.

The present invention provides an optical film, comprising: atransparent supporting substrate; a structuralized layer including aplurality of prisms integrally arranged on the transparent supportingsubstrate along their longitudinal directions; wherein the extendedtrajectories of the prisms vary with a first change mode on theirlongitudinal directions and the heights of the prisms vary with a secondchange mode along their longitudinal directions.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to sufficiently understand the essence, advantages and thepreferred embodiments of the present invention, the following detaileddescription will be more clearly understood by referring to theaccompanying drawings.

FIGS. 1 a-1 c illustrate the first kind of change concepts in accordancewith the present invention;

FIGS. 2 a-2 c are further examples illustrating the first kind of changeconcepts in accordance with the present invention;

FIGS. 3 a-3 d are an embodiment using the first kind of change conceptsin accordance with the present invention;

FIGS. 4 a-4 e illustrate the second kind of change concepts inaccordance with the present invention;

FIGS. 5 a-5 d are the first embodiment using the second kind of changeconcepts in accordance with the present invention;

FIGS. 6 a-6 d are the second embodiment using the second kind of changeconcepts in accordance with the present invention;

FIGS. 7 a-7 c illustrate the third kind of change concepts in accordancewith the present invention;

FIGS. 8 a-8 d are an embodiment using the third kind of change conceptsin accordance with the present invention;

FIGS. 9 a-9 c illustrate another kind of change concepts in accordancewith the present invention;

FIGS. 10 a-10 c illustrate another example using the second concepts(FIG. 4);

FIG. 10 d is another optical film carrying out the second concepts (FIG.4) in accordance with the present invention;

FIG. 11 shows the relative positions of the optical film as shown inFIG. 10 d and an LCD panel located;

FIG. 12 shows the relative positions of the optical film as shown inFIG. 10 d and another optical film located;

FIG. 13 shows an actual stereo diagram of the sample 11 in table 1through a microscope;

FIG. 14 shows a measured two-dimensional profile through athree-dimensional contour gauge (Kosaka ET4000a);

FIG. 15 shows a measured three-dimensional profile through athree-dimensional contour gauge (Kosaka ET4000a).

DETAILED DESCRIPTION OF THE INVENTION

The following description shows the preferred embodiments of the presentinvention. The present invention is described below by referring to theembodiments and the figures. Thus, the present invention is not intendedto be limited to the embodiments shown, but is to be accorded theprinciples disclosed herein. Furthermore, that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

An optical film has a structure varying in its height or pitch along alongitudinal direction (prism). The variation includes at least twonormal periods, wherein the first change period is larger than a secondchange period. Furthermore, the variation also cannot be periodic, orcan be defined in a broader change manner, e.g. a random change or achange repeatedly showing two (or more) periods. Therefore, theapplication designates the first variation mode and the second variationmode as a general variation mode. The foregoing normal period mean theaverage of the periodic lengths appearing in the first change mode. Thatis, each periodic length or pitch in the first change mode can bechangeable or constant. For example, the periodic lengths P11, P12, P13,P11, P12, P13 . . . appear in the first change mode. P11, P12 and P13can be the same or totally different. The following embodiments can beexplained according to the foregoing definitions, wherein the firstchange period and the second change period also can be replaced by thefirst change mode and the second change mode. FIGS. 1 a-1 c illustratethe first kind of change concepts in accordance with the presentinvention. The periods of the two changes both appear in a sine wave.The first change period (as shown in FIG. 1 a; or called the firstchange mode) in combination with the second change period (as shown inFIG. 1 b; or called the second change mode) form a composite changeperiod (as shown in FIG. 1 c; or called a composite change mode). Infact, such a composite change mode can be applied to the prism structureof the optical film for varying prisms' height along its longitudinaldirection and/or prisms' pitch (i.e. the center-to-center distance oftwo adjacent prisms is changeable). The first and second change periodscan have a variation toward its lateral direction (the lateral directionindicates a lateral or transversal perturbation occurring in a sine wavealong its wave front but not indicates the lateral side of each prism)in a regular, half-regular, random or quasi-random manner. The firstchange has a nominal period ranging from 50 μm to 1,000 mm, and thesecond change has a nominal period ranging from 50 μm to 1,000 μm. Thenominal period of the first change is 4 times to 1,000 times the nominalperiod of the second change. Or, the nominal period of the first changeis 0.01 times to 1,000 times the nominal period of the second change.

For easy explain and understand the application, an X, Y and Zorthogonal coordinate system (See FIG. 1 c) is used to interpretvariation directions. The X axis direction is transverse to thedirection from the wave crest to the trough of the prism, or can becalled a lateral direction. The Y axis is perpendicular to the X axis,and in general, is the lengthwise direction of the prism. The Z axis isperpendicular to the X and Y axes, i.e. the common height direction ofthe prism. As to the first concept, the height (parallel to the Z axis)of the prism varies along the lengthwise direction (parallel to the Yaxis direction) of the prism. The variations reflecting both of thefirst and second change periods occur in the Z axis along the Y axis.That is, the height (Z axis) of the prism has the composite variationincluding the first and second change modes along the lengthwisedirection (Y axis) of the prism. The pitches between the prisms areconstant and parallel.

FIGS. 2 a-2 c are further examples illustrating the first kind of changeconcepts in accordance with the present invention. Two change periodsare shown in the form of a square wave. The first change period (FIG. 2a) is combined with the second change period (FIG. 2 b) to form acomposite change period (FIG. 2 c). Practically, the composite variationcan be applied to the prism structure of an optical film to change theheight H and/or pitch P of the prism along its lengthwise direction. Thefirst and second change periods can have a variation toward its lateraldirection in a regular, half-regular, random or quasi-random manner. Thefirst change has a nominal period ranging from 50 μm to 1,000 mm, andthe second change has a nominal period ranging from 50 μm to 1,000 mm.The nominal period of the first change is 4 times to 1,000 times thenominal period of the second change. Or, the nominal period of the firstchange is 0.01 times to 1,000 times the nominal period of the secondchange. Furthermore, the first and second change periods can bereflected on any waveforms such as a sine wave, a square wave, a sawwave, a triangle wave and so on.

FIGS. 3 a-3 d are an embodiment using the first kind of change conceptsin accordance with the present invention. FIG. 3 a shows a brightnessenhancement film 30 of the present invention. FIG. 3 b is a top view ofthe brightness enhancement film 30 in FIG. 3 a. FIG. 3 c is a front viewof the brightness enhancement film 30 in FIG. 3 a. FIG. 3 d is across-sectional view of the brightness enhancement film 30 in FIG. 3 ataken along the crest line of a prism. The first and second changeperiods can be reflected in a square waveform for varying the height ofthe prism structure. The nominal period PR1 of the first change is 4times to 400 times the nominal period PR2 of the second change. Or, thenominal period PR1 of the first change is 0.01 times to 1,000 times thenominal period PR2 of the second change. The pitch between the prisms ispreferably between 10 μm to 200 μm, and more preferably between 24 μm to60 μm. The average amplitude of the first change period ranges from 1 μmto 5 μm. That is, the total height of the structure of the prism 31varies in a range of 1 μm to 5 μm. The included angle between thestructuralized surfaces of the prism 31 can be any degree in a range of70 degrees to 110 degrees, more preferably between 80 degrees to 100degrees. In this embodiment, such prism structure can decrease thecontact area between the structuralized surfaces of the prism film(brightness enhancement film) so as to reduce the occurrence of opticalcoupling. As shown in FIG. 3 b, the brightness enhancement film 30comprises a transparent supporting substrate 33 and a structuralizedlayer 32. The structuralized layer 34 is integrally formed on thetransparent supporting substrate 33.

FIGS. 4 a-4 e illustrate the second kind of change concepts inaccordance with the present invention. The height (Z axis) of the prismvaries along the lengthwise direction (Y axis) and the lateral direction(X axis) of the prism. The first change period (FIG. 4 a) has avariation in the Z axis along the X axis, and the second change period(FIG. 4 b) has a variation in the Z axis along the Y axis. The prismsare parallel to each other and their pitch is constant.

FIGS. 5 a-5 d are the first embodiment using the second kind of changeconcepts in accordance with the present invention. And, FIGS. 6 a-6 dare the second embodiment using the second kind of change concepts inaccordance with the present invention. The first and second changeperiods are all reflected on the height of the prism structure. If otherembodiments illustrate a non-periodical or random variation, the firstand second change periods can be called or replaced by first and secondchange modes. The present invention is not limited to this embodiment.The nominal period of the first change is 4 times to 400 times thenominal period of the second change. Or, the nominal period of the firstchange is 0.01 times to 1,000 times the nominal period of the secondchange. The prisms are parallel to each other and their pitch isconstant. The pitch of the brightness enhancement film 50 (prisms 51)between its prisms is preferably between 10 μm to 200 μm, and morepreferably between 24 μm to 60 μm. The average amplitude of the firstchange period ranges from 2 μm to 10 μm, and the average amplitude ofthe second change period ranges from 1 μm to 5 μm. That is, the totalheight of the structure of the prism 51 varies in a range of 1 μm to 15μm. The included angle between the structuralized surfaces of the prism51 can be any degree in a range of 70 degrees to 110 degrees, morepreferably between 80 degrees to 100 degrees. In this embodiment, suchprism structure can decrease the contact area between the structuralizedsurfaces of the prism film (brightness enhancement film) so as to reducethe occurrence of optical coupling.

FIGS. 7 a-7 c illustrate the third kind of change concepts in accordancewith the present invention. The first change period (FIG. 7 a) iscombined with the second change period (FIG. 7 b) to form a compositechange period (FIG. 7 c). The first change period can be applied to theprism structure of an optical film for varying the pitches (X axis)between the prisms along their lengthwise direction (Y axis). The secondchange period can be applied to the prism structure of an optical filmfor varying the heights (Z axis) of the prisms along their lengthwisedirection (Y axis). In this embodiment, the height (Z axis) of the prismvaries along its lateral direction (X axis). The first change period hasa variation in the X axis along the Y axis, and the second change periodhas a variation in the Z axis along the Y axis. The first change has anominal period ranging from 50 nm to 1,000 mm, and the second change hasa nominal period ranging from 50 nm to 1,000 mm. The nominal period ofthe first change is 4 times to 1,000 times the nominal period of thesecond change. Or, the nominal period of the first change is 0.01 timesto 1,000 times the nominal period of the second change.

FIGS. 8 a-8 d are an embodiment using the third kind of change conceptsin accordance with the present invention. The first change period isreflected on the pitches of the prism structure, and the second periodis reflected on the heights of the prism structure. The nominal periodof the first change is 4 times to 400 times the nominal period of thesecond change. The pitch between two prisms is constant and they areparallel to each other. The pitch between the prisms is preferablybetween 10 μm to 200 μm, and more preferably between 24 μm to 60 μm. Theaverage amplitude of the first change period ranges from 2 μm to 10 μm,and the average amplitude of the second change period ranges from 1 μmto 10 μm. The included angle of the prism structure can be any degree ina range of 70 degrees to 110 degrees, more preferably between 80 degreesto 100 degrees. In this embodiment, such prism structure can decreasethe contact area between the structuralized surfaces of the prism film(brightness enhancement film) so as to reduce the occurrence of opticalcoupling (wet out). The prism structure of the present embodiment canminimize the possibility of seeing optical interference fringes (moire;or moire fringes).

In another change concept (not shown in figures) of the presentinvention, the first and second change periods can all be reflected onboth of the pitch and height of the prism structure. In other words, thefirst and second change periods are applied to vary the pitch (X axis)and the height (Z axis) along either the Y axis or the X axis. FIGS. 9a-9 c illustrate another kind of change concepts in accordance with thepresent invention. The first change period (FIG. 9 a) is combined withthe second change period (FIG. 9 b) to form a composite change period(FIG. 9 c). According to the first change period, the pitch (X axis) andthe height (Z axis) of the prism structure vary along the lengthwisedirection (Y axis). Further, according to the second change period, theheight (Z axis) of the prism structure varies along the lengthwisedirection (Y axis). In the embodiment, the height (Z axis) of the prismsimultaneously varies along the lateral direction (X axis) and thelengthwise direction (Y axis). The first change period has a variationin the X axis and the Z axis along the Y axis, and the second changeperiod has a variation in the Z axis along the Y axis. The first andsecond change periods can have a variation toward its lateral directionin a regular, half-regular, random or quasi-random manner. The firstchange has a nominal period ranging from 50 μm to 1,000 mm, and thesecond change has a nominal period ranging from 50 μm to 1,000 mm. Thenominal period of the first change is 4 times to 1,000 times the nominalperiod of the second change. Or, the nominal period of the first changeis 0.01 times to 1,000 times the nominal period of the second change.

FIGS. 10 a-10 c illustrate another example using the second concepts(FIG. 4). The first change period has a variation (FIG. 10 b) in the Zaxis along the X axis, and the second change period has a variation(FIG. 10 a) in the Z axis along the Y axis. Both of the first and secondchange periods are reflected on the variation of the height of theprism. The lengths of the first and second change periods can be randomvalues. The nominal period of the first change is 0.01 times to 400times the nominal period of the second change. FIG. 10 d is an opticalfilm of the present invention, i.e. another embodiment of the brightnessenhancement film (BEF or prism sheet).

As shown in the table 1 below, the experiment is conducted based on theforegoing change concepts. The defects of the optical interferencefringes and wet out (BEF to LCD panel) are inspected from a 47 inches TVwhose resolution is 1920×1080. The relative positions of an optical film110 and an LCD panel 111 arranged are shown in FIG. 11. The numeralreference 110 indicates the brightness enhancement film mentionedtherein. The extending direction of the prism is parallel to thelongitudinal edge of the LCD panel. A light source is incident to theback surface of the optical film and is emitted from the prism surface,and then enters into the LCD panel 111. The defects of the opticalinterference fringes and wet out can be seen on the LCD panel 111 byeyes, and are resulted from the optical film 110. As shown in FIG. 12,another kind of the wet out defect (BEF to BEF) is resulted from abrightness enhancement film 112 disposed above the foregoing opticalfilm, and can be or cannot be inspected by eyes.

Referring to the Table 1, it shows the optical film has the greatability to avoid the wet out effect not only for the BEF to LCD but alsofor BEF to BEF. Particuly, the sampleb 5, 9, and 12 have the bestresults on the resistance to the optical interference fringes. Theexisting optical interference fringes are not visual.

TABLE 1 First change Second change period period Prism Nominal NominalNominal Period Amplitude Period Amplitude Period Wet out Wet out Sample(μM) (μM) (μM) (μM) Angle (μM) moiré (BEF-panel) (BEF-BEF) 1 337 10 20003 90° 33 YES NO NO 2 284 10 2000 3 90° 33 YES NO NO 3 766 10 2000 3 90°33 YES NO NO 4 931 10 2000 3 90° 33 YES NO NO 5 942 7.5 2000 3 90° 38 NONO NO 6 816 6 2000 3 90° 33 NO NO NO 7 820 5 2000 3 90° 50 YES NO NO 8200 3 2000 3 90° 50 YES NO NO 9 620 5 2000 3 90° 50 Slight NO NO 10 11205 2000 3 90° 50 YES NO NO 11 676 7.5 2000 3 90° 38 NO NO NO 12 1132 7.52000 3 90° 38 NO NO NO *YES represnts the defects is visible; NOrepresents the defects are invisible; Slight represents the defects areslightly visible.

FIG. 13 shows an actual stereo diag ofe sample 11 in table 1 through amicroscope. Through the microscope, the nominal period of the firstchange period can be identified as the total width of 17 prisms (nominalpitch is 38 μm), and hence, the length of the nominal period is around676 μm. FIGS. 14 and 15 shows measured two-dimensional andthree-dimensional profiles through a three-dimensional contour gauge(Kosaka ET4000a). FIG. 14 is a two-dimensional profile by scanning alongthe X direction. This figure shows that the height of the prim randomlyvarμs (about 3 μm). A very high prism appears for each interval of 676μm. The very high prism has a variation of about 3 μm in its height, andhence, the length of the first change nominal perifore sample 11 is 676μm and its amplitude is 7.5 μm. Moreover, the length of the secondchange nominal period is 2,000 nm and its amplitude is 3 nm. And, thelength of the first change period is 0.338 times that of the secondchange period. FIG. 15 shows a measured three-dimensional profilethrough a three-dimensional contour gauge (Kosaka ET4000a).

The foregoing embodiments of the invention have been presented for thepurpose of illustration. Although the invention has been described bycertain preceding examples, it is not to be construed as being limitedby them. They are not intended to be exhaustive, or to limit the scopeof the invention. Modifications, improvements and variations within thescope of the invention are possible in light of this disclosure.

1. An optical film, comprising: a transparent supporting substrate; astructuralized layer including a plurality of prisms integrally arrangedon the transparent supporting substrate along their longitudinaldirections; wherein a height of at least one of the prisms varies alongits longitudinal direction and a variation in the height includes afirst change mode and a second change mode; wherein a trajectorygenerated from the second change mode is superposed on and along atrajectory generated from the first change mode.
 2. The optical filmaccording to claim 1, wherein the first and second change modes have avariation in a regular, half-regular, random or quasi-random manner. 3.The optical film according to claim 1, wherein the first change mode hasa first change nominal period and the second change mode has a secondchange nominal period, wherein the first change nominal period is 0.01times to 1,000 times the second change nominal period.
 4. The opticalfilm according to claim 3, wherein a length of the first change nominalperiod ranges from 50 μm to 1,000 mm and the first change nominal periodhas an average amplitude ranging from 2 μm to 10 μm.
 5. The optical filmaccording to claim 3, wherein a length of the second change nominalperiod ranges from 50 μm to 1,000 mm and the second change nominalperiod has an average amplitude ranging from 1 μm to 10 μm.
 6. Theoptical film according to claim 1, wherein the trajectories generatedfrom the first change mode and the second change mode show a sine wave,a square wave, a saw wave or a triangle wave.
 7. The optical filmaccording to claim 1, wherein pitches between the prisms are between 10μm to 200 μm.
 8. The optical film according to claim 7, wherein thepitches between the prisms are between 24 μm to 60 μm.
 9. The opticalfilm according to claim 1, wherein an included angle between thesurfaces of the prism is within a range of from 70 degrees to 110degrees.
 10. The optical film according to claim 9, wherein the includedangle between the surfaces of the prism is within a range of from 80degrees to 100 degrees.
 11. An optical film, comprising: a transparentsupporting substrate; a structuralized layer including a plurality ofprisms integrally arranged on the transparent supporting substrate alongtheir longitudinal directions; wherein pitches between the prisms varywith a first change mode and heights of the prisms vary with a secondchange mode along their longitudinal directions.
 12. The optical filmaccording to claim 11, wherein the first and second change modes have avariation in a regular, half-regular, random or quasi-random manner. 13.The optical film according to claim 11, wherein the first change modehas a first change nominal period and the second change mode has asecond change nominal period, wherein the first change nominal period is0.01 times to 1,000 times the second change nominal period.
 14. Theoptical film according to claim 13, wherein a length of the first changenominal period ranges from 50 μm to 1,000 mm and the first changenominal period has an average amplitude ranging from 2 μm to 10 μm. 15.The optical film according to claim 13, wherein a length of the secondchange nominal period ranges from 50 μm to 1,000 mm and the secondchange nominal period has an average amplitude ranging from 1 μm to 10μm.
 16. The optical film according to claim 11, wherein trajectoriesgenerated from the first change mode and the second change mode show asine wave, a square wave, a saw wave or a triangle wave.
 17. The opticalfilm according to claim 11, wherein the pitches between the prisms arebetween 10 μm to 200 μm.
 18. The optical film according to claim 17,wherein the pitches between the prisms are between 24 μm to 60 μm. 19.The optical film according to claim 11, wherein an included anglebetween the surfaces of the prism is within a range of from 70 degreesto 110 degrees.
 20. The optical film according to claim 19, wherein theincluded angle between the surfaces of the prism is within a range offrom 80 degrees to 100 degrees.
 21. An optical film, comprising: atransparent supporting substrate; a structuralized layer including aplurality of prisms integrally arranged on the transparent supportingsubstrate along their longitudinal directions; wherein extendedtrajectories of the prisms vary with a first change mode on theirlongitudinal directions and heights of the prisms vary with a secondchange mode along their longitudinal directions.
 22. The optical filmaccording to claim 21, wherein the first and second change modes have avariation in a regular, half-regular, random or quasi-random manner. 23.The optical film according to claim 21, wherein the first change modehas a first change nominal period and the second change mode has asecond change nominal period, wherein the first change nominal period is0.01 times to 1,000 times the second change nominal period.
 24. Theoptical film according to claim 23, wherein a length of the first changenominal period ranges from 50 μm to 1,000 mm and the first changenominal period has an average amplitude ranging from 2 μm to 10 μm. 25.The optical film according to claim 23, wherein a length of the secondchange nominal period ranges from 50 μm to 1,000 mm and the secondchange nominal period has an average amplitude ranging from 1 μm to 10μm.
 26. The optical film according to claim 21, wherein the trajectoriesgenerated from the first change mode and the second change mode show asine wave, a square wave, a saw wave or a triangle wave.
 27. The opticalfilm according to claim 21, wherein pitches between the prisms arebetween 10 μm to 200 μm.
 28. The optical film according to claim 27,wherein the pitches between the prisms are between 24 μm to 60 μm. 29.The optical film according to claim 21, wherein an included anglebetween the surfaces of the prism is within a range of from 70 degreesto 110 degrees.
 30. The optical film according to claim 29, wherein theincluded angle between the surfaces of the prism is within a range offrom 80 degrees to 100 degrees.
 31. The optical film according to claim21, wherein the heights of the prisms vary with the first change modealong their longitudinal directions and a trajectory of the heightreflecting the second change mode is superposed on a trajectory of theheight reflecting the first change mode.
 32. A liquid crystal displaycomprising an optical film according to claim
 1. 33. A backlight moduleof a liquid crystal display comprising an optical film according toclaim 1.